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    I Acceleration Versus Gravity (General Relativity)

    Hi, It is usually claimed that a person in an accelerating elevator with an acceleration equals to the gravity of the earth; this person cannot make any experiment that makes him know whether he is in the elevator or on the surface of earth. However, if this person project two light beams...
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    I Determinism of Macroscopic World

    Hi, If the microscopic world is not deterministic, then naturally the macroscopic world should also be not deterministic because it is based on the microscopic world. The problem is that the macroscopic world is deterministic; the evidence of this is that a lot of our technology and...
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    Synchronous Sources and Interference

    Hi, What is the scientific definition of "synchronous sources" when we talk about the conditions of interference? Thank you.
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    Hooke's law for vibrating massive spring

    Hello, Just for curiosity... Is Hooke's law valid for a vibrating massive spring ? I have done some calculations using both Newton's 2nd Law and the conservation of energy to a horizontal swinging spring connected to a small block in the absence of any friction. I have found that the tension of...
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    Doppler Effect and Conservation of Energy

    Hi, Consider a person who holds a light source of frequency f. Consider an observer who sees this light. The energy of each photon as received by the observer is E = hf. The observer is now fixed. Now suppose that the person who holds the light source starts to move towards the observer, then...
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    Quantum Fluctuations and Big Bang

    Hi, I have read that quantum fluctuations have created our universe through the Big Bang. The issue that I didn't understand is that as far as I know quantum fluctuations are properties of space. How could these fluctuations exist before the Big Bang while there was no space before the Big...
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    Focal Length of a Plano Convex Lens

    Hi, The focal length of the Plano-Convex lens as calculated by the lens maker equation: Does it work when light crosses the lens from the convex side, planar side, or both? Thanks.
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    Diffraction accompanied with photoelectric effect

    Hi, I have read that light could either behave as waves or particles but not both at the same time. What will happen if we perform the diffraction experiment, but replace the screen with a metal whose work function is small enough so electrons could be ejected from the metal? Do we see...
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    Proof that zeta(-1) is -1/12

    Hi, I have read that zeta(x) = 1^(-x) + 2^(-x) + 3^(-x) + ... infinity for x = -1, zeta(-1) = 1 + 2 + 3 + 4 + 5 ... What confused me is that zeta(-1) = -1/12 and so 1 + 2 + 3 + 4 + 5 + ... = -1/12 Can anybody give a proof that zeta(-1) is -1/12. Thanks in advance.
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    Newton's 3rd Law and electric charges

    Hi, Is Newton's 3rd law applicable to electric charges? I have a problem with a situation where two identical positive charges moving perpendicular to each other. The magnetic forces exerted by each one on the other have same magnitude but their directions are not opposite.
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    Scope of physics

    Hi, I have finished 2 years studying physics, and 1 year studying teaching physics. From the first lecture in the university, students are taught that physics is an experimental science. The strength of physics is that it could be verified experimentally. If someone has some doubts...
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    Law VS Principle VS Theory

    Hi, I tried to google it, but I got much more confused with different and contradictory definitions. Can somebody please help me understand the difference between these terms? Is there overlapping between them? Some books say "Law of conservation of energy", but others say "Principle of...
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    Bernoulli and Continuity Equations

    Hi, Consider a vertical relatively long cylinder of constant radius open at both ends. We fill this cylinder with water and prevent water from falling down by a certain sheet as seen in the figure. Now suppose we remove the sheet suddenly. Let v1 be the speed of the upper surface of...
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    Spring stretched at both ends

    Hi, Consider a spring of stiffness k and mass m. If we stretch - simultaneously - both ends of the spring by force F1 and F2. What will be the elongation of the spring? Can I add the forces and say that x = (F1 + F2)/k ?
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    Person exits simultaneously in two different positions?

    Hi, Consider two fixed points A and B in a frame S. In this frame a person P moves towards the point A and does an event EA. After that, this person moves to the point B and does an event EB. Based on Lorentz's transformations, it is very easy to find a frame S' where events EA and EB...
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    Relativity of Simultaneity Issue

    Hi, Consider an observer S pushing a fork towards a material x-axis as seen in the figure below According to this observer E and F cut x-axis simultaneously. Now consider an observer S' moving along x-axis with a constant relativistic speed. It is very easy, using Lorentz transformation, to...
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    Velocity Vector of Light

    Hi, Consider a source of light transmitting light at an angle θ with x-axis as seen in the following figure: Now an observer looking at x-axis will determine the speed of light to be c.cos θ, and the one looking at y-axis determine c.sin θ. How can we resolve this according to the...
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    Measuring contracted length

    Consider an observer equipped with a good ruler and a rod. This observer measures the length of the rod and finds it 30 cm. Now this observer with his ruler and rod rides a space ship and moves with a high speed. He then tries to measure the length of the rod using his ruler. According the SR...
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    Kirchoff's voltage law with coils

    There is an idea that Kirchoff's voltage law could not be applied to circuit containing coils like this link. What makes me confused is that almost all books apply Kirchoff's voltage law to RL and RLC circuit.
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    Index of refraction and density

    Hi, Is it possible that the medium of lesser density has larger index of refraction? If yes, please give me some examples. Thanks in advance.
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    Force and special relativity.

    Hi, Does the magnitude of the force change when the frame moves with a high speed with respect to another frame at rest? If yes, How? Thanks for any replies.
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    Hooke's Law of a spring with a mass

    Hi, I know that for a massless spring the tension is T = kx, where k is a constant and x is the compression or elongation. What is the expression of the tension if the spring has a mass m? Thanks in advance.
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    Condition of the work energy theorem

    Hi, After reading several resources about work and energy, I am confused about the conditions that should be satisfied in order to be able to apply work energy theorem. It seems that textbooks have different arguments about this issue. I can summarize what textbooks say in three different...
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    Complex number paradox

    Hi, How can we explain this paradox: Thanks in advance.
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    Charging capacitor by another capacitor

    Hi, Consider a charged capacitor connected to a an empty capacitor. The charged capacitor starts to discharge while the empty capacitor starts charging. The whole process finishes when the two capacitors maintain the same potential difference. The problem is that when calculating the energies...
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    Direction of friction in rolling.

    Hi, Consider a ball rolling upward without slipping on an inclined plane. What is the direction of the force of static friction? Let me explain what confuses me. I know that the friction opposes the tendency of the motion. If we consider the whole motion of the ball as upward, then the...
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    Two slits diffraction vs interference

    Hi, What is the difference in the setups of the Young's double slit experiment and the two slits diffraction? I found textbooks discuss each differently, yet I found no difference in the setup. Thanks to any help.
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    Torque, point or axis of rotation

    Hi, 1) Do we calculate torque with respect to a point or with respect to an axis? I have read them both in different resources, and so I am confused!! 2) If we calculate torque with respect to an axis, many introductory textbooks discuss the motion of the gyroscope by considering how...
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    Simple horizontal harmonic oscillator with spring that has a mass.

    Hi, Consider a block of mass M connected to a spring of mass m and stiffness k horizontally on a frictionless table. We elongate the block some distance, and then release it so that it now oscillates. According to the theoretical study using energy methods, we see that the mass of the...
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    Why oil and water do not mix?

    Hi, Why do oil and water not mix? We know that some liquids mix together while others not, I am looking for the reason of this. Thanks to any replies